Installation for the very long storage of products that emit a high heat flux

ABSTRACT

An installation for very long term storage of products emitting a high thermal flux.  
     The installation comprises a container ( 14 ), in which the products to be stored are placed, and an evaporator ( 22 ) surrounding the container, to evacuate the heat through a heat-pipe effect. The evaporator ( 22 ) comprises a jacket ( 28 ), pipes ( 32 ) integral with the jacket and filled with a coolant fluid such as water, and a system for tightening the evaporator ( 22 ) on the container ( 14 ). The arrangement is such that the evaporator ( 22 ) is not maintained in close contact with the container ( 14 ) except in front of the pipes ( 32 ). Advantageously, channels ( 42 ) are provided for air circulation by natural convention between the evaporator ( 22 ) and the container ( 14 ), on both sides of the pipes ( 32 ).

TECHNICAL FIELD

The present invention relates to a storage installation, that is to saystorage under surveillance and reversible, for a very long time period(more than 50 years), of calorific products emitting a high thermalflux.

Such a storage installation can, in particular, be used for very longterm storage of nuclear waste such as irradiated nuclear fuels. Thestorage of such products requires temperature control of the containersin which they are placed.

STATE OF THE ART

Very long term storage of calorific products such as nuclear waste isusually carried out by processing the waste in containers and thenplacing the latter in cavities made in the ground with limits defined byconcrete walls.

The high thermal flux generated by the calorific products must beevacuated by a cooling system to stabilise the surface temperature ofthe containers. This makes it possible to ensure the stability of thecontainer structures and the calorific products they hold. This alsomakes it possible to ensure the stability of the concrete of thesurrounding walls. Preferably, the cooling systems are passive.

In the document FR-A-2 791 805, a very long term storage installation isproposed for calorific products. In this installation, the thermal poweris extracted as close as possible to the sealed barrier represented bythe container, without intrusion and in a passive manner, before beingevacuated from the site to the exterior by a non-contaminable coolingcircuit.

More precisely, this document proposes surrounding each containertightly, over the whole of its external cylindrical surface, with aflexible and removable jacket consisting, for example, of a tightenedand stapled thin metal sheet surrounding the container in such a waythat the smooth external surfaces of the container and the jacket arenormally in contact. The application of the jacket on the externalsurface of the container is ensured by tightening at several pointsduring closure (or stapling) of the jacket.

Externally the jacket is equipped, at regular intervals (for exampleabout 20 cm), with vertical pipes of either circular or squarecross-section. These pipes are intimately linked to the jacket, from athermal conduction point of view, in such a way as to form an evaporatorfor the coolant fluid. Preferably, this fluid functions in bi-phaseliquid-vapour regimen and constitutes a heat pipe with the circuit inwhich it is confined. The heat pipe condenser is set outside the site,where heat exchange takes place with the free air circulating by naturalconvection.

In this known installation, the transmission of the thermal flux fromthe container is ensured, on the one hand, by direct contact of thecontainer walls and the metal sheet forming the jacket and, on the otherhand, by contact between said metal sheet and the pipes it supports.

According to another embodiment described in document FR-A-2 791 805,the pipes are integral with the jacket sections, themselves assembledend to end by welding or by any other mechanical connection means. Inthis case, the thermal efficiency of the system depends only on thequality of the contact between the containers and the juxtaposed jacketsections.

In all cases, the quality of heat transfer rises when the contactresistance falls, that is to say when the contact between surfaces isclosest. In other terms, good heat flux transfer between the containerand the flexible jacket surrounding it depends on the thickness of theresidual air film between the two walls being limited to a fraction of amillimetre.

A cooling supplement is usually brought by the surrounding air, inconstant natural convection at the external surface of the heat pipejacket. In order to ensure cooling if there is an incident or anaccident, means for producing forced convection movement of air can beprovided. The heat transfer increases with the external surface of thejacket, when the latter is made of a heat-conducting material and whenthe contact resistance between the container and the jacket is low.Furthermore, in a preferred embodiment, the pipes can be provided withcooling fins in order to increase the transfer surface between thejacket and the surrounding air and to provide a longer period of timefor intervention in the case of accident.

Modelling and then experiments carried out on scale 1, on containers of2 metres diameter, resulted in obtaining the performance described indocument FR-A-2 791 805.

Continuation of this work and its orientation towards industrialisationrevealed the difficulty of obtaining an average play of less than 0.3 mmbetween the containers and the jacket surface. Such precision,obtainable on a prototype, is difficult to reproduce on an industrialscale with traditional tools and any attempt to reduce the play, forexample to 0.1 mm, raises manufacturing costs enormously. But, thisaverage play constitutes the most important parameter for theperformance of installations.

DESCRIPTION OF THE INVENTION

The aim of the invention is a very long term storage installation forcalorific products, comparable to the installation described in FR-A-2791 805 but whose original design enables at least comparableperformances to be obtained in a much simpler and less costly manner,using traditional industrial means.

According to the invention, a very long term storage installation forcalorific products is proposed, comprising at least one confinementcontainer for said products, an evaporator comprising a jacketsurrounding the container and a plurality of pipes integral with thejacket and filled with a coolant fluid, and means for tightening theevaporator on the container, characterised in that the evaporator has aninternal surface such that the tightening means keep the evaporator inclose contact with an external surface of the container only in front ofeach of the pipes.

Design studies and modelling of such an installation together with testsconcerning certain sensitive characteristics such as the interfacebetween the jacket and the container showed that limiting the contactsurfaces between the container and the jacket to restricted zones infront of the pipes made it possible to obtain, with traditionalindustrial means and therefore at reasonable cost, just as efficientheat transfer between the container and the pipes as that which wouldhave been obtained, in the installation according to prior art shown bydocument FR-A-2 791 805 when setting a constant average play between thecontainer and jacket at about 0.1 mm, which is very difficult to obtainindustrially.

Advantageously, the internal surface of the evaporator, between thepipes, has a radius of curvature that is substantially higher than thatof the external surface of the container.

Preferably, since the contact zone between the container and each of thepipes has a well defined surface and is not limited to a line,particularly in the case of pipes with circular cross-section, theinternal surface of the evaporator comprises, in front of each of thepipes, a part with shape complementary to the external surface of thecontainer, maintained in close surface contact with said externalsurface by tightening means.

According to a first embodiment of the invention, the pipes are fixed,preferably by welding, inside a continuous structure, of almost circularcross-section, forming the jacket. In this case, the pipes can includecooling fins, located between the jacket and the container.

According to a second embodiment of the invention, each pipe consists ofa single piece with two jacket sections, and the neighbouring pipesections are assembled together edge to edge to form the jacket. Theneighbouring pipe sections can then be assembled either by welding or byany mechanical connection means whatsoever.

The pipes can have either a substantially square or rectangularcross-section, or a substantially circular cross-section. In the lattercase, advantageously the pipes have flanges with an internal facemaintained in close surface contact against the external surface of thecontainer by the tightening means.

As an option, an external surface of the evaporator can include coolingfins.

Finally, according to a particularly advantageous improvement of theinvention, outside the zones located in front of the pipes, theevaporator is separated from the container in such a way as to definevertical channels for air circulation, by natural convection. In avariant of an embodiment of the invention, the channels are then part ofa closed circuit constituting a supplementary confinement barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, as illustrative and non-limiting examples, various embodiments ofthe invention will be described, with reference to the attacheddrawings, in which:

FIG. 1 is a vertical cross-section representing very diagrammatically apart of a storage installation for calorific products according to theinvention;

FIG. 2 is a horizontal cross-section illustrating diagrammatically apart of an evaporator according to the invention, in quasi-linearcontact with a container set in the installation;

FIG. 3 is a view comparable to FIG. 2, showing diagrammatically the caseof an evaporator in surface contact with a container holding calorificproducts;

FIG. 4 is a cross-section comparable to FIGS. 2 and 3, representing anevaporator according to a first embodiment of the invention in greaterdetail, and the associated tightening means;

FIG. 5 is a cross-section comparable to FIG. 4, showing side-by-sidethree variants of possible cross-sections for the evaporator pipes, aswell as the presence of optional cooling fins on the jacket;

FIG. 6 is a cross-section comparable to FIGS. 4 and 5, showing anothervariant of the first embodiment of the invention;

FIG. 7 is a cross-section comparable to FIGS. 4 to 6, showingside-by-side three variants of a second embodiment of the invention;

FIG. 8 shows three curves illustrating the evolution of the averagetemperature (in ° C.) within the thickness of a container holding acalorific product, in function of the average play (in mm) between theevaporator and the container, respectively in the case of constant play(curve A), in the case of contact between the pipes (curve B) and in thecase of contact in front of the pipes according to the invention (curveC);

FIG. 9 shows the distribution of thermal flux (in W/m²) in function ofthe distance (in mm) from the axis of a pipe, in the direction of thecircumference of the container, respectively in the case of constantplay of 0.01 mm (curve D), in the case of constant play of 0.3 mm (curveE) and in the case of contact in front of the pipes and an average playof 0.3 mm (curve F), and

FIG. 10 shows the evolution of the maximum temperature of the container(in ° C.) in function of the tightening force applied on the evaporator(in Newton).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS ACCORDING TO THE INVENTION

In FIG. 1, part of an installation according to the invention is showndiagrammatically, intended for very long term storage of calorificproducts such as nuclear waste consisting, for example, of irradiatednuclear fuels.

In its general configuration, this installation is comparable to thatdescribed in document FR-A-2 791 805. For more details, this documentcan well be consulted.

In order to understand the invention fully, it is simply noted here thatthe installation comprises a closed cavity 10, defined laterally andtowards the bottom by concrete walls 12. The dimensions of the cavity 10are such that one or several containers 14 can be housed, in which thenuclear wastes to be stored are processed. The containers 14 have theshape of cylindrical drums and they are placed in the cavity 10 withtheir axes oriented closely vertical. There is a space 16 between eachcontainer 14 and the walls 12 of the cavity 10 to allow circulation ofthe surrounding air, by natural convection. Thus, the container 14 restson the base of the cavity 10 on top of a pedestal 17.

The cavity 10 is closed at the top by a concrete slab 18, including aremovable plug 20 on top of each of the containers 14.

In order to ensure evacuation of the heat emitted by the processednuclear waste in the containers 14 by a passive method, meaning withouta supply of external energy, a heat pipe is associated with eachcontainer. More precisely, this heat pipe comprises an evaporator 22surrounding the container 14, an air condenser 24 placed above the slab18 and two ducts 26 linking the evaporator 22 to the air condenser 24through the plug 20. The air condenser 24 can be common to severalcontainers 14.

A cooling fluid such as water at 100° C. is placed in the heat pipe. Thephase changes of this fluid (evaporation/condensation) in the heat pipeensure transfer of the heat emitted by the nuclear waste from the hotsource constituted by the container 14 to the cold source constituted bythe air condenser 24.

As shown diagrammatically in FIG. 2, the evaporator 22 comprises ajacket 28, closely surrounding the totality of the external peripheralsurface 30 of the container 14, and a plurality of pipes 32 integralwith the jacket 28. The pipes 32 are parallel to each other and also tothe closely vertical axis of the container and they are spaced in asubstantially regular fashion at equal distances from each other, aroundthe whole periphery of the container.

With reference again to FIG. 1, it can be seen that the pipes 32 arelinked to an annular distributor of liquid water 34 at their lower endsand in an annular collector of vaporised water 36 at their upper ends.The distributor 34 and the collector 36 are linked separately to the aircondenser 24 by one of the ducts 26 and the latter comprise removableconnections 38, below the plug 20. The pipes 32 as well as thecollectors 34 and 36 are filled with the cooling fluid contained in theheat pipe.

The evaporator 22 is mounted on the container 14, in a removable way, bytightening means 40, and an example will be described below withreference to FIG. 4.

According to the invention and as illustrated diagrammatically in FIG.2, the internal surface of the evaporator 22, that is the surface of theevaporator facing the container 14, is produced in such a way that thetightening means 40 maintain the evaporator 22 in close contact with theexternal surface 30 of the container 14 only in front of each of thepipes 32. Thus, the parts of the jacket 28 that are in place between thepipes 32 are separated from the external surface 30 of the container 14,in such a way as to form vertical channels 42, of closely uniform orvariable thickness, between the jacket 28 and the container 14. Thesechannels 42 constitute a sort of chimney generating air circulationaround the container 14, by natural convection.

This air circulation can be mainly laminar or turbulent, according tothe specific power dissipated by the container, the height of thecontainer and, to a lesser degree, by the diameter of the container. Theturbulent character of the flow improves the cooling of the container.It is encouraged by a specific thermal power equal to or greater than 1kW/m² and by an increase in the height of the container and the radialthickness of the vertical channels 42.

Tests were carried out with specific thermal powers ranging from 1 kW/m²to over 3 kW/m² and, more particularly, around 2.5 kW/m². The heightswere comprised between 2 m and 5 m, the greatest height improving theefficiency of heat transfer. To ensure that the circulation in thevertical channels 42 has significant efficiency, the radial thicknessmust be greater than 1 cm; this is the reason why the tests were carriedout preferably with radial thicknesses comprised between 4 cm and 12 cm.

For annular geometry, the production of a chimney effect by naturalconvection is controlled by three parameters as follows:

the height of the chimney; in the present case the height of the chimneyis between 5 and 6 metres when the container is filled with irradiatedfuels, which generates a very strong draught. Nonetheless, a height of 1metre corresponding to a container filled with hot objects of shorterlength produces the same proportional efficiency;

the presence of the cylindrical container generating the thermal flux:the container is an excellent generator of thermal flux; this flux canbe considered to be homogeneous on the cylindrical wall, and

the width of the annular space ΔR between the container and the jacket,for a given diameter; in the present case, the width of the annularspace 42 alone is not sufficient for defining the convection in thisgeometry; thus the relationship between the radii R1 of the containerand R2 of the jacket must be taken into account.

The air movement is caused by the variation of volumic mass of the fluidsubmitted to a force field. The grouping governing the naturalconvection is the Grashof number Gr, but the correlations generallyallowed bring in the intervention of the Rayleigh number.

For a container diameter of around 2 metres, calculations demonstratethat the chimney effect begins to develop from ΔR=1 cm. The effect thenincreases with ΔR to reach an optimum value of about 5 to 6 cm (thedefinition of this optimum value depends here on maximum utilisation ofa high yield heat pipe evaporator, coupled with a high performancecooling system by natural convection). This optimum value corresponds toan extraction value by natural convection of about 40% of the extractedtotal power (conduction+radiation+natural convection in channels42+external natural convection). With ΔR=4 cm, the percentage of powerextracted by the chimney type effect is about 25 to 30% of the total.This value was validated experimentally on a model of 2 m diameter, 1.5m height and a thermal flux of 2.5 kW/m². The value ΔR=4 cm correspondsto the external dimensions of a square tube of 40 mm×40 mm whoseinternal cross-section is needed for stable operation in diphase siphonmode (passive mode).

Beyond ΔR=about 6 to 7 cm, the chimney type effect does not increase anymore, and it tends to decrease towards natural convection in free spacefor ΔR>10 cm.

These values are justified in a coupled situation of power extractionboth by the heat pipe (to take greatest advantage of extraction byconduction) and by the natural chimney convection.

The gain in performance of the system, subject of the present inventionis, optimally, about 20%. For equal generated power in the container,this results in a significant lowering of the skin temperature of thecontainer by about. 10 to 20° C. (depending on the nature of thedifferent materials) and for thermal fluxes of 2 to 3 kW/m². Thisaddition is therefore very significant.

As shown diagrammatically in FIG. 2, the contact between the evaporator22 and the container 14 can be limited to quasi-linear zonescorresponding to the generatrix lines of the container 14 situated atright angles to each of the pipes 32.

In order to improve the heat exchange even further, the internal surfaceof the evaporator 22 can also comprise, on the right of each of thetubes 32, a part 44, of limited width, whose shape is complementary tothat of the external surface 30 of the container 14, as shown in FIG. 3.Application of the tightening means 40 (FIG. 4) then has the effect ofmaintaining these parts 44 in close surface contact with the externalsurface 30 of the container 14.

The quasi-selective contact of FIG. 2 like the surface contact of FIG. 3can be obtained by providing the internal surface of the evaporator 22,between the pipes 32, with a radius of curvature greater than that ofthe external surface 30 of the container 14. Thus, as a non-limitingexample, in the case of a container with a radius of 1000 mm, the partsof the evaporator 22 located between the pipes 32 can have a radius ofabout 1200 mm. The maximum play between the evaporator and the containeris then, for example, 0.85 mm. In the case of quasi-selective contactsuch as that shown in FIG. 2, an average play of about 0.45 mm isobtained inside the channels 42.

In a first embodiment according to the invention shown in FIG. 4, thejacket 28 takes the form of a continuous structure, of closely circularcross-section and of small thickness, surrounding the container 14 at adistance. This structure is constituted, for example, of metal sheet.The pipes 32 are then fixed inside the jacket 28 by any appropriatemeans. Advantageously, this fixation is ensured by welded points.

FIG. 4 also shows a possible embodiment of the tightening means 40.

As shown in FIG. 4, the evaporator 22 is open along a generatrix andcomprises two opposite edges 22 a, oriented parallel to the axis of thecontainer 14. The tightening means 40 are set between the two edges 22a. More precisely, the tightening means 40 comprise a plurality of bolts46, that cross the holes formed in the parts 48, set along the edges 22a of the evaporator, on its outwards facing surface. A helicoidalcompression spring 50 is mounted on each of the bolts 46, in such a wayas to maintain the tightening force substantially constant in thehypothesis of possible differential dilatations between the container 14and the evaporator 22.

FIG. 5 shows different variants together of the first embodiment of theinvention described with reference to FIG. 4. In practice, it isunderstood that these variants are alternative solutions, generallyimplemented separately from each other, apart from contrary indications.

The different variants shown in FIG. 5 relate first of all to the shapeof the pipes 32. Thus, in any case, these pipes can have a circular,square or rectangular cross-section, that is to say flattened in thedirection of their thickness. The thermal evacuation is increasinglyefficient when the contact surface between the container and the partsof the evaporator located in front of the pipes increases, that is tosay changing from pipes of circular cross-section to pipes ofrectangular cross-section. Nonetheless, the extent of this contactsurface must remain sufficiently low so that close contact can beobtained without difficulty.

As a non-limiting illustration, the pipes 32 can be set every 200 mm andhave a cross-section of 40×40 mm or 60×60 mm, in the case of squarepipes.

As shown in the right-hand part of FIG. 5, the heat exchange betweenpipes 32 and the air circulating in the annular spaces 42 can beimproved by equipping the pipes with cooling fins 32 a, located betweenthe jacket 28 and the container 14. These fins 32 a can be added ontothe pipes 32 of any cross-section shape whatsoever or can be made in asingle piece with said pipes, under the form of extruded profiles.

As shown in FIG. 6, in the case where pipes of circular cross-sectionare used, the heat exchange can be improved by equipping each of thepipes 32 with flanges 52, on the side of the container 14. The internalface of the flanges 52 is then maintained in close surface contactagainst the external surface 30 of the container 14.

In FIG. 7, different possible variants are shown for an evaporatoraccording to a second embodiment of the invention.

In this second embodiment, the jacket 28 and pipes 32 are made in asingle piece. More precisely, each of the pipes 32 is made in a singlepiece with two sections 28 a of the jacket 28. Each of the sections 28a, in cross-section in a horizontal plane, has the shape of an arc of acircle whose length is equal to half the length of the jacket betweentwo consecutive pipes 32. The sections 28 a of the neighbouring pipes 32are assembled together edge to edge, following the generatrix lines ofthe container 14, to form the jacket 28.

Edge to edge assembly of the sections 28 a can be ensured either bywelding 54 or by mechanical connection means 56, such as fish joints orother, as shown in FIG. 7.

When the pipes 32 have a circular cross-section, they can compriseflanges 52, as described above with reference to FIG. 6, within theframework of the first embodiment according to the invention. Theflanges 52 are then constituted of an internal face with a shapecomplementary to that of the external cylindrical shape of the container14. In this case, the tightening means associated with the evaporatorkeep the internal face of each of the flanges 52 in tight surfacecontact, meaning without play, against the external surface of thecontainer 14.

It is also shown in FIG. 7 that each of the parts in a single piececonsisting of a pipe 32 and two jacket sections 28 a can also compriseone or several cooling fins 58 on its surface facing outwards, that isaway from the container 14. In the first embodiment according to theinvention, shown in FIGS. 4 to 6, such cooling fins 58 (FIG. 5) can alsobe envisaged. In this case, the fins 58 are added by welding them ontothe external surface of the metal sheet forming the jacket 28.

In the second embodiment according to the invention, the tighteningmeans can be similar to those used in the first embodiment, such asdescribed above with reference to FIG. 4.

Modelling of finished elements made by the applicant showed,surprisingly, than an evaporator 22 with limited surface contact withthe container 14 (corresponding to a play of 0.01 mm), at right anglesto the heat pipe tubes 32, according to the invention, makes it possibleto obtain thermal properties essentially identical to those obtained byusing an evaporator according to the prior art described in the documentFR-A-2 791 805, in which a uniform play of 0.1 mm is obtained over thewhole interface between the evaporator and the container. This result isparticularly advantageous from an industrial point of view because it ismuch easier to ensure limited local contact at right angles to the pipes32 than to obtain a uniform play of 0.1 mm over the entire surface ofthe evaporator 22.

These results are shown in FIG. 8, which represents an orthonomicreference on which the abscissae show the average play (in mm) betweenthe evaporator 22 and the container 14 and the ordinates show theaverage temperature (in ° C.) in the thickness of the container 14. Moreprecisely, curve A corresponds to the case of an evaporator of priorart, in which constant play is envisaged between the evaporator and thecontainer, curve B corresponds to the case of an evaporator intended tobe in contact locally with the container only between the pipes, andcurve C corresponds to the case of an evaporator 22 in accordance withthe present invention, meaning in local contact with the container 14only in front of the pipes 32.

As Table 1 below also demonstrates, it can be seen that the efficiencyof the heat pipe depends essentially on the play under the pipes 32 andonly to a small degree on the average play between the evaporator 22 andthe container 14. For example, if the maximum temperature of thecontainer is fixed at 155° C., it can be seen from Table 1 that thisresult can be obtained with an average play of 0.5 mm and a contact infront of the pipes 14 according to the invention. This result iscomparable to that obtained in the case of a uniform play of 0.1 mmaccording to prior art, which is very difficult to obtain. TABLE 1Average temperature inside the container (in ° C.) Average UniformContact in Contact play (mm) play front of pipes between pipes 0.01 1380.05 140 150 0.1 153 0.3 175 149 186 0.5 193 155 203 1 224 3 283

The presence of an average play of 0.5 mm with contact between theevaporator 22 and the container 14 in front of the pipes 32, accordingto the invention, means that the play is nil at right angles to thepipes 32 (that is to say, equal to 0.01 mm in the case of the modelling)and that it evolves linearly up to 1 mm in the middle of the arc of acircle formed in cross-section by the evaporator between twoneighbouring pipes 32. Such an arrangement is perfectly practicable withtraditional industrial means. In fact, for equal thermal yield, it makesit possible to multiply the average play by five on condition that thecontact zones are localised in front of the pipes 32.

As described with reference to FIGS. 2 and 3, the contact zones can bequasi-linear or, preferably, can take the shape of narrow surfacesextending over the whole height of the container.

In FIG. 9, the evolution of thermal flux (in W/m²) is shown in functionof the distance from the axis of a pipe 32 (in mm), on the arc of acircle formed in cross-section by the evaporator 22. More precisely,this evolution is shown by D in the case of a constant play of 0.01 mmbetween the evaporator 22 and the container 14, by E in the case of aconstant play of 0.3 mm and by F in the case of a linear contact infront of the pipes 32 and an average play of 0.3 mm.

It can be seen from FIG. 9 that the distribution of thermal flux closelydepends on the nature of the play between the evaporator and thecontainer. In particular, it can be noted that the major part of thethermal flux is transferred in the zones close to the pipes 32 and thatthis phenomenon is accentuated when the play under the pipes diminishes.Thus, in the case of a constant play of 0.3 mm, half the thermal flux istransferred in 31 mm starting from the pipes (curve E), whereas thisdistance falls to 18 mm in the case of a constant play of 0.01 mm (curveD) and to 17 mm in the case of linear contact under the pipes with anaverage play of 0.3 mm (curve F). The results shown in FIG. 9 thusconfirm the interest of privileging contacts at right angles to thepipes 32 in accordance with the invention. These results were confirmedexperimentally using a model for thermal testing.

By replacing the linear contacts under the pipes 32 by surface contacts,this phenomenon was accentuated. Consequently, it is then no longer halfbut the totality of the thermal flux that is transferred under the pipes32.

The influence of tightening forces applied to the evaporator 22 by thetightening means 40 was also studied. The results of this study areshown in FIG. 10. This figure represents the evolution of the maximumtemperature of the container (in ° C.) in function of the tighteningforce (in Newton). It can be seen that the temperature falls when thetightening force is increased from 0 to 4000 N, but that beyond 4000 Nany increase in tightening force has no effect. Tightening means 40 suchas those described with reference to FIG. 4 make it possible to reachthe value of 4000 N without any particular problem.

An evaporator 22 according to the invention, produced by combining theprinciple of quasi-linear contact of FIG. 2 with the second embodimentdescribed with reference to FIG. 7 (jacket sections 28 a and pipes 32 ina single piece), was first tested with the numerical values indicatedabove with reference to FIG. 2 (container of 1000 mm radius, evaporatorof radius of curvature equal to 1200 mm, maximum play of 0.85 mm,quasi-linear contact under the pipes). The experiment confirmed thatthis evaporator was thermally equivalent to a prior art evaporator withan average play of 0.01 mm relative to the container, which is verydifficult to obtain in practice.

After this, an evaporator 22 was produced combining the characteristicsof FIG. 3 (surface contact) and the second embodiment of the presentinvention. In this case, the contact surface at right angles to thepipes 32 must not be too wide, because of the risk of encountering theproblems of implementation characteristic of prior art. Thus it appearsthat, for a container 14 of 2000 mm diameter, contact zones of from 40to 60 mm wide constitute a good compromise between obtaining greatlyincreased thermal yield and uncomplicated production.

Since the biggest part of the jacket 28 takes part only very partiallyin the passage of thermal flux, the first embodiment described abovewith reference to FIGS. 4 to 6 constituted a third experimental stage.In fact, this embodiment makes it possible, at reduced cost, to maintainan acceptable thermal yield. By placing the jacket 28 at a distance fromthe container 14 equal to the external dimensions of the pipe 32, allthe manufacturing tolerances disappear. The jacket 28 forms a continuouscircular structure making it possible to gird the pipes 32 and to holdthem on the container 14.

Furthermore, an annular space, crown shaped, is created between thejacket and the container. This space corresponds to the channels 42 ofFIG. 2. It encourages the development of a sort of chimney effect,allowing the ambient air thus channelled to circulate vertically underthe effect of natural convection, whose drive is the thermal power ofthe container 14. Thus very efficient independent passive cooling isproduced, since it results from direct contact with the container. Thiscooling effect adds up to that of the heat pipe in contact with thecontainer. The total yield of this embodiment is therefore greater thanthat of prior art, for a much lower cost.

It can be considered that the natural convection of the air outside thejacket 28 is not affected significantly and that this phenomenon isadded to the two preceding phenomena.

Such turbulence in the vertical channels 42 is so efficient that it canreduce the thermal flux to be evacuated by the fluid circuit. Thisreduction is advantageous in two cases: on the one hand, if anaccidental failure affects the fluid circuit, the delay available forcarrying out an intervention is much longer; on the other hand, over thelong term, the date of ceasing utilisation of this fluid circuit takinginto account the reduction of thermal flux is advanced significantly.

A variant of an embodiment according to the invention consists ofextracting the air circulating in the vertical channels 42 in closedcircuit, using means known to those skilled in the art. Furthermore,this variant has the advantage of producing a sealed barrier forsupplementary confinement, raising security in the case of a possibleaccident situation, and avoiding affecting the storage air thermally.

Above all, it is to be noted that the jacket 28 also acts as a screenvis-à-vis the concrete structures on the site and that its temperatureis lower than that of the jacket used in prior art since it is cooled onits two faces and is not in thermal continuity with the pipes 32.

Finally it is also observed that, because of the high performance of theinstallation according to the invention, the thermal conductivity of thematerials used contributes very little to the thermal yield. Thus thedesigner has a much wider choice of materials than in prior art.

1. Installation for very long term storage of calorific products,comprising at least one confinement container (14) for said products, anevaporator (22) comprising a jacket (28) surrounding the container (14)and a plurality of pipes (32) integral with the jacket (28) and filledwith a coolant fluid, and means (40) for tightening the evaporator (22)on the container (14), characterised in that the evaporator (22) has aninternal surface such that the tightening means (40) maintain theevaporator (22) in close contact with an external surface (30) of thecontainer (14) only in front of each of the pipes (32).
 2. Installationaccording to claim 1, wherein the internal surface of the evaporator(22) has, between the pipes (32), a radius of curvature considerablygreater than that of the external surface (30) of the container (14). 3.Installation according to either one or the other of claims 1 and 2,wherein the internal surface of the evaporator (22) comprises, in frontof each of the pipes (32), a part (44) of shape complementary with theexternal surface (30) of the container (14), maintained in close surfacecontact with said external surface by tightening means (40). 4.Installation according to any one of claims 1 to 3, wherein the pipes(32) are fixed inside a continuous structure, of substantially circularcross-section, forming the jacket (28).
 5. Installation according toclaim 4, wherein the pipes (32) are fixed inside the jacket (28) bywelding.
 6. Installation according to claim 4, wherein the pipes (32)comprise cooling fins (32 a) located between the jacket (28) and thecontainer (14).
 7. Installation according to any one of claims 1 to 3,herein each pipe (32) is made in a single piece with two jacket sections(28 a) and the jacket sections (28 a) integral with neighbouring pipes(32) are assembled together edge to edge to form the jacket (28). 8.Installation according to claim 7, wherein the jacket sections (28 a)integral with neighbouring pipes (32) are assembled together by welding(54).
 9. Installation according to either one or the other of claims 7and 8, wherein the jacket sections. (28 a) integral with neighbouringpipes (32) are assembled together by mechanical connection means (56).10. Installation according to any one of claims 1 to 9, wherein thepipes (32) have a substantially square or rectangular cross-section. 11.Installation according to any one of claims 1 to 9, wherein the pipes(32) have a substantially circular cross-section.
 12. Installationaccording to claim 10, wherein the pipes (32) have flanges (52) with oneinternal face maintained in close surface contact against the externalsurface (30) of the container (14) by tightening means (40). 13.Installation according to any one of claims 1 to 12, wherein an externalsurface of the evaporator (22) comprises cooling fins (58). 14.Installation according to any one of the above claims wherein, apartfrom zones located in front of the pipes (32), the evaporator (22) is ata distance from the container (14) so as to define vertical channels(42) for air circulation, by natural convection.
 15. Installationaccording to claim 14, wherein the channels (42) are part of a closedcircuit constituting a confinement.